We propose, using histological, ultrastructural, electrophysiological, and biochemical techniques, to study trophic exchanges of proteins between nerve axons and their perikaryon, surrounding glial cells, and pre- or post-synaptic structures. In particular, we wish to examine the cellular mechanisms which enable medial giant and lateral giant axons in the CNS of crayfish to survive for many months when severed from their cell body. From our preliminary data, we expect that severed medial giant axons survive in large part because of glial-neuronal protein exchange. Conversely, we expect that severed lateral giant axons survive in large part because of inter-neuronal exchange of proteins at regions that were electrotonic junctions when the neurons were intact. We expect that, compared to vertebrates, it will be much easier to demonstrate the nature and functional significance of trophic exchanges of proteins in these CNS axons because of their large size and because of their highly developed exchange mechanism. Nevertheless, we expect that vertebrate axons use qualitatively similar cellular mechanisms for protein exchange and that results from our model systems will be directly applicable to vertebrates.